The present application relates to a fuel system, and more particularly to an aircraft suction fuel system.
A rotary-wing aircraft fuel system communicates fuel from the aircraft fuel tanks to the engine packages. Various components within the fuel system maintain each fuel line section outside the fuel tank in suction such that the static pressure in the fuel line segments outside the tank are maintained below the ambient air pressure. In such conventional suction fuel feed systems, an engine fuel pump alone draws fuel from the fuel tank to the engine package. With such a suction feed system, should a fuel line outside the fuel tank develop a leak or be ruptured due to ballistic damage, air is drawn into the fuel line rather than a fuel leak or spray out of the fuel line which would occur if the fuel line was positively pressurized.
While desirable, conventional suction fuel feed systems may have inherent limitations, often due to the volatility of aviation fuels. Aviation fuels tend to outgas fuel vapor and air dissolved in the fuel when the fuel is exposed to a significant suction pressure. The higher the magnitude of the suction pressure acting on the fuel, the more vapor and air dissolved in the fuel will outgas from the fuel. This results in gaseous bubbles in the fuel line. This effect generally increases with increasing fuel temperature and aircraft altitude as the pressure of the fuel vapor and air which outgases from the fuel becomes an increased percentage of the total pressure inside the fuel line.
The fuel vapor and air that outgas from the liquid fuel result in a two-phase liquid-gas flow in the fuel line, generally in the form of bubbles, or, in extreme cases, alternating slugs of air/vapor and liquid fuel in the fuel line. Engine fuel pumps used in suction fuel feed systems are designed to tolerate a certain amount of gaseous air/vapor at the pump inlet, normally expressed as a maximum vapor-to-liquid ratio (V/L) capability of the pump. If the V/L of the fuel at the engine fuel pump inlet exceeds the V/L capability of the pump, the pump will cavitate and will not pump fuel until the excess V/L condition is removed. This may subsequently, depending on the length of time the condition exists, result in engine flame-out.
In rotary-wing aircraft suction fuel feed systems, fuel pressure at the engine package fuel inlet is reduced relative to the fuel pressure in the fuel tank due to the pressure drop in the fuel line. The pressure drop in the fuel line is generally the sum of the dynamic pressure losses and the head loss. The dynamic pressure losses result from fuel flow through the fuel line and may include, for example, frictional losses, bend losses, expansion losses, contraction losses, etc. Dynamic losses increase with increased fuel flow. In conventional suction fuel feed systems, head loss is based on the vertical height of the engine fuel pump inlet above the fuel level in the fuel tank. Head loss is highest when the fuel tank is at minimum fuel levels. During aircraft maneuvers, the normal 1−g head loss is multiplied by the number of g's experienced.
One challenge in the design of rotary-wing aircraft fuel systems is to maintain the pressure drop in the fuel line low enough to maintain fuel V/L at the engine fuel pump inlet within the engine fuel pump V/L capability throughout the aircraft flight envelope. If design constraints of the fuel line, such as the line height, length, maximum allowable line diameter and high-g capability preclude operation within the maximum V/L limit over a portion of the flight envelope with the engine fuel pump alone drawing fuel from the fuel tank, some form of pumping assistance may be required.
The conventional suction fuel system approach to such pumping assistance is through operation of one or more airframe-supplied fuel boost pumps, either within the fuel tank or in the fuel line section between the tank and the engine package. Such pumping assistance has conventionally caused a situation in which, when one or more boost pumps are turned on, some or all of the fuel line between the fuel pump and the engine package is positively pressurized at some operating conditions and the benefits of a suction fuel system may be lost in some or all of the fuel line.